You pretty much always draw it first, so you have a better reference. Drawing a character from scratch is easier than sculpting it, so there's less of a chance you horribly fuck the proportions up or something like that. This applies to most 3D modelling: If you can, get a reference, even if you make it yourself on a sheet of paper.

I like to sculpt my characters right away. If drawing them helps get a better idea of what you want, do that. If you like the sketch you can make extra sure to stick to it, or you can change it. There's no "right way" to do it.

The only "wrong way" i can think of is box-modelling a character from scratch without sculpting first - it's not like you can't, but it's backwards and way harder to make it look good.